L-Glutamic acid has been hitherto industrially produced by a fermentative method using coryneform bacteria belonging to the genus Brevibacterium or Corynebacterium.
Recently, it has been revealed that a mutant strain of Escherichia coli, in which the .alpha.-KGDH activity is deficient or lowered, and the glutamic acid-decomposing activity is lowered, has high L-glutamic acid productivity (Japanese Patent Laid-open No. 5-244970).
On the contrary, it was reported that a mutant strain having lowered .alpha.-KGDH activity had approximately the same L-glutamic acid productivity as that of its parent strain in the case of a bacterium belonging to the genus Brevibacterium (Agric. Biol. Chem., 44, 1897 (1980), Agric. Biol. Chem., 46, 493 (1982)). Therefore, it has been believed that the level of .alpha.-KGDH activity is not important for production of L-glutamic acid in coryneform bacteria.
On the other hand, it was found that a mutant strain of a L-glutamic acid-producing bacterium belonging to the genus Brevibacterium having lowered .alpha.-KGDH activity produces L-glutamic acid at high efficiency (maximum yield of 53%) when the bacterium is cultivated in a medium which contains a material containing an excessive amount of biotin as a carbon source without addition of materials which suppress an effect of biotin such as penicillins and surface active agents (Japanese Patent Laid-open No. 6-23779). However, since it has been believed that the level of .alpha.-KGDH activity is not important for production of L-gultamic acid in the coryneform bacteria as described above, there has been no example in which an .alpha.-KGDH gene of a coryneform L-glutamic acid-producing bacterium is cloned and analyzed. Further, mutant strains of coryneform bacteria being completely deficient in .alpha.-KGDH have been unknown.